linux-kernel - Re: [PATCH] powerpc/code-patching: Relax verification of patchability

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Message-ID: <5a995d46-5292-b6e1-f88e-81d8cbc40b83@csgroup.eu>
Date:   Fri, 26 Nov 2021 09:49:00 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Sachin Sant <sachinp@...ux.vnet.ibm.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH] powerpc/code-patching: Relax verification of patchability



Le 26/11/2021 à 08:39, Christophe Leroy a écrit :
> Commit 8b8a8f0ab3f5 ("powerpc/code-patching: Improve verification of
> patchability") introduced a stricter verification of the patched
> area by checking it is proper kernel text.
> 
> But as least two usages of patch_instruction() fall outside:
> - Code patching selftests, which use stack and vmalloc space.
> - Ftrace
> 
> So for the time being, partially revert commit 8b8a8f0ab3f5 and add
> a onetime warning:
> 
>    Running code patching self-tests ...
>    patch_instruction() called on invalid text address 0xe1011e58 from test_code_patching+0x34/0xd6c
> 
> Reported-by: Sachin Sant <sachinp@...ux.vnet.ibm.com>
> Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
> Cc: Nicholas Piggin <npiggin@...il.com>
> Fixes: 8b8a8f0ab3f5 ("powerpc/code-patching: Improve verification of patchability")
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
>   arch/powerpc/lib/code-patching.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
> index 1dd636a85cc1..c87eea773930 100644
> --- a/arch/powerpc/lib/code-patching.c
> +++ b/arch/powerpc/lib/code-patching.c
> @@ -190,9 +190,13 @@ static int do_patch_instruction(u32 *addr, struct ppc_inst instr)
>   int patch_instruction(u32 *addr, struct ppc_inst instr)
>   {
>   	/* Make sure we aren't patching a freed init section */
> -	if (!kernel_text_address((unsigned long)addr))
> +	if (system_state >= SYSTEM_FREEING_INITMEM && init_section_contains(addr, 4))
>   		return 0;
>   
> +	if (!kernel_text_address((unsigned long)addr))
> +		pr_warn_once("%s() called on invalid text address 0x%p from %pS\n",
> +			     __func__, addr, __builtin_return_address(0));
> +

May it be better to use pr_warn_ratelimited() instead in order to catch 
more than the first occurence ?

>   	return do_patch_instruction(addr, instr);
>   }
>   NOKPROBE_SYMBOL(patch_instruction);
>